Shooting in the wind is part guesswork, part science and totally frustrating for newer shooters. As with any other skill, it takes a lot of practice to become a good shooter in the wind. A quick tour of YouTube and a Google search will yield several ways to get on target, but for this post, we’re going to provide a rule of thumb for a specific cartridge and a way for you to “calibrate” 1 mil of deflection in a 10 mph wind. We hope the 10:1 helps not only the newer shooters but adds another useful tool for experienced shooters.
Who is this post for?
This post is aimed at new to intermediate shooters. Experienced hands can already rhyme off their wind calls and do quick math to achieve their ballistic solutions, but for newer shooters we wanted to give them some simple tools to help them so they can focus on delivering a great shot. If you're an experienced shot and you are coaching the next generation, perhaps there is something here to pass along.
Image 1: How a wind formula appears to new shooters.
So what is 10:1?
If you know your baseline 1 mil hold in a 10 mph wind at a known distance, you’ll be able to quickly determine your deflection if the wind is more or less than 10 mph and/or if the distance is more or less than your baseline. This is useful when you do not have a spotter and are in a target-rich environment involving multiple targets at a variety of ranges. Basically, there is no time to do the math or reach for your solver.
Why 10:1? Because these numbers are easy to work with and remember. As you’ll see, this is all about math and once you see how we calibrate a 1 mil wind hold with a 10 mph wind, you’ll have the tools to create your own baseline hold using any numbers you prefer.
While this post deals with wind, we’re not going to talk about how to read the wind, we’ll look at ways to assess wind speed in a future post. For this post we’re interested in providing shooters with a baseline wind deflection hold and hopefully raise your interest level to get you searching out answers for various types of wind/distances/ammo types.
Image 2: The internet's most commonly sourced wind value clock artwork
The concept of wind
We’re assuming the reader understands a few very basic concepts, namely that wind will influence a bullet in flight. If you are on the range, imagine your target is at 12 o’clock and you are in the centre of that clock. Any wind coming directly from 3 or 9 o’clock will push your in-flight bullet left or right respectively. The wind that affects your bullet is considered to be “full value” wind. This means that a 10 mph wind from exactly 3 o’clock (right to left) will be a 10 mph full value wind. If that 10 mph wind is coming from 5 o’clock then your bullet will be influenced less. That 10 mph wind from 5 o’clock is “half value” instead of full value. Therefore, the effect on the bullet from a 10 mph half value wind is 5 mph. In this case our wind calculations would utilize 5 mph as the “full value” instead of 10 mph despite the 10 mph wind.
Where does full value and half value wind start and stop on this imaginary clock? From 2:30 to 3:30 and 8:30 to 9:30 are the directions we assign full value to. That means if we are receiving wind from within that range on the clock, whatever that wind is, it’s full value. Winds from 11, 1, 5 and 7 o’clock are half value while 12 and 6 o’clock winds are zero value.
The short version of a mil
When we refer to a “mil”, we’re talking mil-radians which is an angular measurement. 1 mil represents 10cm at 100m, 20cm at 200m and so on. If you have a reticle in your scope beyond a basic duplex, you probably have lines or dots (or both) in your reticle to show you these mils. These mil lines or dots are probably separated by smaller lines representing portions of a mil. There are many different reticles out there so make sure you understand yours. On the topic of scopes, if you have a mil reticle in either a first focal plane (FFP) or even a second focal plane (SFP), this whole process will be pretty simple, but don’t forget to use full zoom on your SFP scope for your holds and also when you are judging distance using mils (another future post). If your scope uses something other than mils, all is not lost, a bit of research and some practice may yield some useful baseline numbers.
What about my turrets?
You don’t have to worry about dialing for wind because wind isn’t constant. That’s right leave that windage turret alone. In dynamic shooting environments where distances between targets may vary and appear and disappear quickly, there is no time to dial elevation changes either, you’ll do holdovers for those different distances. Trust your reticle! If you’ve spent good money on your glass you can trust those lines and dots and for the most practical distances you won’t need to touch either turret.
How to calculate a 10:1
10:1 is our way of determining where our impacts on target are relative to a 1 mil hold in a 10 mph full value wind. To use a common example, let’s take a .308 and see where the shot falls when faced with a 10 mph wind to a 1 mil correction. We can figure this out one of two ways: using “cowboy math” and by using a ballistic solver. Let’s start with a bit of fifth-grade math and then we’ll confirm our answers with an app. We’ll explore cowboy math in greater detail in a future post, but for now let’s consider the 10:1 and cowboy math workable out to about 500 metres (some modification of the formula will be necessary beyond 600 – again, that’s for a future post).
Components of cowboy math
Range (in metres and represented by shortening the range to a decimal; 500 metres would be represented by .5)
Wind (full value in miles per hour represented in its whole number; 10 mph would simply be 10)
Ammo rating (the mph “rating” of your cartridge. As an example, a .308 is usually said to have a 4 mph rating, you might need to try different numbers to work with your ammo especially if you are cooking up custom loads)
Here’s a sample formula step by step for a .308 in a 10 mph full value wind at 300 metres to see how much deflection we need:
10 (mph full value wind) / 4 (ammo rating) x .3 (range)
10 / 4 x .3 = correction in mils
2.5 x .3 = .75 mils holdover
Okay, so we’ve got a 10 mph wind but at 300 metres it looks like our deflection for wind is .75 mils. That’s good info and you should take note of it, but we’re still trying to find what distance we’ll require for 1 mil hold, not a .75 mil hold. Let’s try again and adjust the distance to 400 metres.
10 (mph full value wind) / 4 (ammo rating) x .4 (range)
10 / 4 x .4 = correction in mils
2.5 x .4 = 1 mil
There we have it! Our .308 needs a 1 mil hold at 400 metres in a 10 mph full value wind. We’ll now refer to this as .308/400/10/1. Make a note of this so you will have your baseline reference available to start working from. From these numbers you can now make wind calls at different distances and with different wind speeds. As an example, if you are confident with your .308/400/10/1 and you’ve proven it on the range, you can then reasonably (and quickly) deduce that a 200 metre target in 10 mph full value winds will require .5 mils correction. Also, a 5 mph wind at 400 metres can use a .5 mils correction and so on.
Common question: So, you are telling me that a formula combining Imperial variables and a modified metric measurement value will accurately result in a wind deflection hold result in mils out to these practical distances?
Our answer: Yes and stop overthinking it.
Validating the math and your .308/400/10/1 baseline
If you can’t get to the range, you can always compare your math with the output of a good ballistic solver.
Here are some screen grabs from the Hornady app. It’s free and it works well. In the app I created a .308 / 168gr BTHP flying at 2700 fps from a 10:1 twist rate barrel. The zero was 100m, the atmospherics were left at the default settings and line of sight to bore was 1.77 inches. The wind was input as full value left to right (from 9 o’clock) at 10 mph. The outputs show mils (mil-radians).
400m, 300m and 200m - 10 mph full value wind from 9'oclock
You might notice a very slight difference in corrections in your app of choice if you compare a 9 o'clock wind to a 3 o'clock wind, this is because of the clockwise spin of the bullet (have any of you ever used a rifle with a left hand twist barrel?). A left to right full value wind will usually require a smidgen (practically immeasurable by the shooter) more correction than the same wind speed in a right to left full value scenario. Because we’re practical shooters, we’re not concerned with those differences nor would we be able to do any additional math in a dynamic shooting environment. We’ll talk more about bullets in flight in future posts.
Have you checked your math against your app? You might need to experiment with the ammo rating value from the cowboy math equation to make it work for your cartridge. If you are cooking up hotter .308 rounds in your basement, you’ll need to use a higher mph rating. These kinds of exercises are how we like to spend our time with ballistic solvers, we can run simulations to see if our cowboy math is working for a particular type of round. Can you figure out your baseline 1 mil hold for a 6.5 Creedmoor or a .30-06? Does the math work as nicely for your calibre of choice as it does for 308?
If you are new to the sport and you've read this far, congrats! We're always curious and despite years behind the guns we never stop learning and the modern tools have shortened the learning curve dramatically. Let us know if these posts are helpful, we have a few more in mind. Stay safe and shoot straight!